The present invention relates to the field of gas turbines, and more particularly to air-cooled turbine blades
Modern gas turbines work at extremely high temperatures. This requires intensive cooling of the turbine blades. A particular difficulty is to reliably cool the exposed regions of the blades. One of these regions is the shroud band or the -shroud-band elements of the blade. German Patent No. DE 198 13 173 A1 or U.S. Pat. No. 5,785,496 disclose turbine blades of this general type. One possibility of cooling shroud-band elements has been described in the publication DE 198 13 173 A1 mentioned at the beginning. In this publication, it is proposed (see FIGS. 3 and 4 there) to cool the shroud-band elements by a number of parallel cooling bores, which extend from the (central) moving blade through the shroud-band element to the outer edge of the shroud-band element and open there into the exterior space.
However, this known solution has the following disadvantages:
Laterally abutting joints between two shroud-band elements of adjacent blades (as can be seen, for example, from FIG. 3 of U.S. Pat. No. 5,482,435) will at least partly close the orifices of the cooling bores. This hinders cooling-air discharge and distribution. The shroud-band element is overheated in operation,
The known shroud-band cooling, on account of the laterally arranged orifices, does not change the conditions of flow over the shroud band; that is to say that the pressure and temperature on the top side of the shroud band remain the same. This is also not changed by the fact that, as proposed in U.S. Pat. No. 5,460,486, certain cooling bores open on the underside of the shroud-band element.
The cooling effect is mainly based on the mixing temperature in the shroud-band surroundings, the mixing temperature being lowered by mixing of the discharging cooling air with the hot gas. No measures are taken in the cooling bores in order to intensify the heat transfer between the cooling air and the shroud-band element.
The object of the invention is therefore to provide a turbine blade having an air-cooled shroud-band element, in which turbine blade the abovementioned disadvantages are avoided in a simple manner and which is distinguished by effective cooling of the shroud-band element, in particular on the exposed top side of the shroud-band element.
This object is achieved by this invention.
The basic idea of the invention consists, on the one hand, in directing the cooling bores through the shroud-band element in such a way that considerable heat transfer between the shroud-band element and cooling air is ensured and, on the other hand, in making these bores open into the exterior space in such a way that the cooling air is reliably admitted to the exposed regions of the shroud band and these regions are additionally cooled. This is achieved by virtue of the fact that, starting from the cooling passage in the blade, the cooling bores, in the region of the shroud-band element, run from inside to outside essentially parallel to the direction of movement of the blade tip and in each case open upstream of the outer margin of the shroud-bank element into a surface recess open toward the exterior space.
In this case, in a first preferred embodiment of the invention, recesses, into which the cooling bores open laterally are sunk in the shroud-band element close to the outer margin from the top side. Due to the discharging cooling air mixing with the hot combustion gases which flow over the top side of the shroud-band element, the temperature in this region is effectively reduced and thus overheating of the shroud band is avoided. In this way, uniform cooling of the shroud-band element over the entire surface is achieved. In addition to effective cooling of the top side of the shroud band, this configuration also has the advantage of very simple manufacture. The discharge of the cooling air on the top side of the shroud-band element is especially effective if, in a preferred development, spaced-apart sealing ribs running parallel to one another are provided on the top side of the shroud-band element and form a cavity in interaction with the opposite casing wall of the gas turbine, and the cooling bores open into this cavity. The discharging cooling air leads to a pressure build-up in the cavity, the result of which is that the penetration of hot gases is reduced.
In another embodiment of the invention, the side edges of the shroud-band elements have recessed portions, into which the cooling bores open. In this case, the recessed portions of opposite shroud-band elements form a gap. During the discharge into the gap, the cooling air splits up into two partial flows. One part flows toward the top side and has the above mentioned effect on the above mentioned cavity between the space-apart sealing ribs. The other part flows toward the underside of the shroud bank and mixes there with the hot gases while setting a mixing temperature which reduces the thermal loading in this region. The ratio of the partial quantities flowing off upward and downward can be influenced by the gap geometry.
In an expedient addition to the invention, it is also proposed that means for improving the heat transfer between cooling air and shroud-band element be provided in the cooling bores. The means for improving the heat transfer at the bore walls may comprise roughness features, ribs and/or turbulence points. In a manner known per se, the bores may be produced by means of the so-called STEM drilling process. Cooling bores having improved heat-transfer properties can be produced in a simple and reliable manner especially by STEM drilling, which has been described, for example, in U.S. Pat. No. 5,306,401 in connection with the production of cooling holes in turbine blades.
Furthermore, improved utilization of the cooling air can be achieved if, in another preferred embodiment of the invention, a choke point for limiting the cooling-air mass flow is provided in each of the cooling bores, and the choke points are each arranged on the inlet side of the cooling bores.